Androgen receptor (AR) belongs to the superfamily of nuclear receptors and is activated by binding to its hormone ligands: androgen, testosterone, or DHT. Upon binding hormone ligand in the cytoplasm, androgen receptor translocates to the nucleus where it binds DNA and functions as a transcription factor to regulate expression of a number of target genes, such as prostate specific antigen (PSA) and TMPRSS2. Knudsen et al. (Trends Endocrinol Metab 21: 315-24, 2010) Bennett et al. (Int J Biochem Cell Biol. 42:813-827, 201).
Androgen receptor (AR) signaling is a critical survival pathway for prostate cancer cells, and androgen-deprivation therapy (ADT), also known as “chemical castration”, is a first-line treatment strategy against hormone-sensitive, androgen-dependent prostate cancer that reduces circulating androgen levels and thereby inhibits AR activity. Although a majority of patients initially respond to ADT, most will eventually develop castrate resistance in which the disease progresses despite castrate levels of testosterone. This type of cancer is known as castrate-resistant prostate cancer (CRPC). There are a number of mechanisms underlying the development of castrate (castration) resistance including an increase in the expression of AR protein which can sensitize cells to low levels of androgen, AR mutations that can alter transactivation or sensitize AR to alternative ligands and the emergence of alternatively spliced forms of AR, which lack the ligand binding domain but can nevertheless act to promote tumour growth in the absence of ligand stimulation. Additionally prostate tumors may also synthesize their own androgens thereby increasing the local intra-tumoral testosterone levels available to activate the AR.
Androgen receptor (AR) signaling is a critical survival pathway for prostate cancer cells, and androgen-deprivation therapy (ADT) remains the principal treatment for patients with locally advanced and metastatic disease. Although a majority of patients initially respond to ADT, most will eventually develop castrate resistance in which the disease progresses despite castrate levels of testosterone. This type of cancer is known as castrate-resistant prostate cancer (CRPC) (Karantos et al., Oncogene advance online: 1-13, 2013). There are a number of mechanisms underlying the development of castration resistance including an increase in the expression of AR protein which can sensitize cells to low levels of androgen (Gregory et al., Cancer Res 61: 2892-2898, 2001; Linja et al., Cancer Res 61: 3550-3555, 2001), AR mutations that can alter transactivation or sensitize AR to alternative ligands (Scher et al., J Clin Oncol 23: 8253-8261, 2005) and the emergence of alternatively spliced forms of AR, which lack the ligand binding domain but can nevertheless act to promote tumour growth in the absence of ligand stimulation (Yingming et al., Cancer Res 73:483-489, 2013). Additionally prostate tumors may also synthesize their own androgens thereby increasing the local intra-tumoral testosterone levels available to activate the AR (Attard et al., Cancer Cell 16:458-462, 2009).
The fact that the androgen receptor remains active in castrate resistant prostate cancer has led to the development of new agents that inhibit the production of androgen ligands or block the actions of these ligands on the AR. These new agents include abiraterone acetate which inhibits 17-α-hydroxylase/17,20-lyase (CYP17) activity resulting in a reduction in residual androgens synthesized by the adrenals and in the prostate tumour itself deBono et al. (N Engl J Med 364: 1995-2006, 2011) and enzalutamide which prevents androgen ligand from binding to AR, translocating to the nucleus, and binding to DNA (Scher et al., N Engl J Med 367:1187-1197, 2012). A number of other androgen synthesis inhibitors or androgen receptor blockers are under development either pre-clinically or clinically and include for example, ARN509, ODM201, TOK001, VT464.
Although the activity of agents such as enzalutamide and abiraterone in CRPC is very encouraging, neither works in all patients and both are associated with the development of additional resistance through re-activation of the AR by the mechanisms described above (Yingming et al., Cancer Res 73:483-489, 2013). Thus, there is a continued need to identify alternative therapies for the treatment of CRPC, and in particular those that can either remove and/or inhibit the activity of all forms of AR including for example, wildtype, mutated and splice variant ARs.
The present invention provides antisense oligonclueotides which by virtue of their design and mode of action (base-pair with the AR RNA target and mediate its destruction by RNase H, an enzyme that destroys the RNA in a DNA/RNA duplex) are aimed at inhibiting the major forms of AR By targeting an appropriate region of the AR mRNA the antisense oligonucleotide will result in inhibition of the major forms (full length, splice variant and mutated forms) of androgen receptor proteins and therefore be suitable for the treatment of patients with CRPC.
Aside from prostate cancer, AR is also implicated as a factor in the progression of other tumours such as breast cancer. In breast cancer AR is expressed in 70-80% of tumours which are also ER positive and in 12% cases which are known as triple negative (no expression of ER, PR and HER2) (Hickey et al., Molecular Endocrinology 26: 1252-1267, 2012). In pre-clinical studies, the androgen receptor antagonist bicalutamide induces anti-proliferative responses in vitro in breast cancer cells and this is potentiated by addition of a Pi3K/mTOR inhibitor (Ni et al., Cancer Cell 20: 119-131, 2011). The 2nd generation anti-androgen, enzalutamide inhibits dihydrotestosterone (DHT) mediated proliferation in ER+/AR+ breast cancer cells and is as effective as tamoxifen at inhibiting estrogen-stimulated breast cancer tumour growth in pre-clinical models in vivo (Cochrane et al., Cancer Res 72(24 Supplement): P2-14-02, 2012). Enzalutamide also inhibits proliferation in HER2+ and triple-negative breast cancer cells. It appears that in situations where estrogen action is reduced (eg. long-term estrogen deprivation or absence of ER) AR levels increase and can become oncogenic. This would suggest that AR antagonists may be best positioned in triple negative or hormone resistant breast cancer settings (Hickey et al., Molecular Endocrinology 26: 1252-1267, 2012). AR targeted therapies are currently under investigation in clinical trials for breast cancer (NCT00468715, NCT01597193, NCT01381874, NCT00755886).
AR is also expressed in a variety of other tumours, including, but not limited to bladder, ovarian, gastric, lung and liver. Pre-clinical data support a similar role as in breast cancer, to promote tumour cell proliferation survival; thus blocking AR in these tumours could have therapeutic clinical benefit (Chang et al., Oncogene advance online: 1-10, 2013).